The organic EL device is a self-emitting device, and has been actively studied for their brighter, superior viewability and ability to display clearer images compared with the liquid crystal device.
In 1987, C. W. Tang et al. at Eastman Kodak developed a laminated structure device using materials assigned with different roles, realizing practical applications of an organic EL device with organic materials. These researchers laminated tris(8-hydroxyquinoline)aluminum (an electron-transporting phosphor; hereinafter, simply Alq3), and a hole-transporting aromatic amine compound, and injected the both charges into the phosphor layer to cause emission in order to obtain a high luminance of 1,000 cd/m2 or more at a voltage of 10 V or less (see, for example, Non-Patent Document 1).
To date, various improvements have been made for practical applications of the organic EL device. In order to realize high efficiency and durability, various roles are further subdivided to provide an electroluminescent device that includes an anode, a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer, and a cathode successively formed on a substrate (see, for example, Non-Patent Document 2).
Further, there have been attempts to use triplet excitons for further improvements of luminous efficiency, and use of phosphorescent materials has been investigated (see, for example, Non-Patent Document 3).
The light emitting layer can also be fabricated by doping a charge-transporting compound, generally called a host material, with a phosphor or a phosphorescent material. As described in the foregoing lecture preprints, selection of organic materials in an organic EL device greatly influences various device characteristics, including efficiency and durability.
In an organic EL device, the charges injected from the both electrodes recombine at the light emitting layer to cause emission. Here, it is important how efficiently the hole and electron charges are transferred to the light emitting layer. The probability of hole-electron recombination can be improved by improving the hole injectability and the electron blocking performance of blocking the injected electrons from the cathode, and high luminous efficiency can be obtained by confining the excitons generated in the light emitting layer. The role of the hole transport material is therefore important, and there is a need for a hole transport material that has high hole injectability, high hole mobility, high electron blocking performance, and high electron durability.
There is also a need for a hole transport material that is stable as a thin film, and has high heat resistance.
Various aromatic amine derivatives are known as the hole transport materials used for the organic EL device (see, for example, Patent Documents 1 and 2). These compounds include a compound known to have an excellent hole mobility of 10−3 cm2/Vs or higher. However, for higher efficiency, a material with higher electron blocking performance, a more stable thin-film state, and higher heat resistance is needed.
There is a report of a high-efficient organic EL device obtained by using a deuterium atom-substituted light emitting layer material (see, for example, Patent Documents 3 and 4).
This is an application of the principle that the luminous efficiency increases by facilitating the formation of excitons when substituted with deuterium atom. While this is true for the material of the light emitting layer, the technique cannot be applied to the material of the hole transport layer. In fact, there is no known example of an application to the material of the hole transport layer.